Command tag queuing refers to queuing multiple commands to a device. Queuing to the device can improve performance because the device itself determines the most efficient way to order and process commands. Devices that support command tag queuing can be divided into two classes: those that clear their queues on error and those that do not. Devices that do not clear their queues on error resume processing of queued commands when the error condition is cleared (either by receiving the next command for NACA=0 error recovery or by receiving a Clear ACA task management command for NACA=1 error recovery). Devices that do clear their queues flush all commands currently outstanding.
Command tag queuing requires the adapter, the device, the device driver, and the adapter driver to support this capability. For a device driver to queue multiple commands to a device (that supports command tag queuing), it must be able to provide at least one of the following values in the scsi_buf.q_tag_msg:
The disk device driver and adapter driver do support this capability. This implementation provides some queuing-specific changeable attributes for disks that can queue commands. With this information, the disk device driver attempts to queue to the disk, first by queuing commands to the adapter driver. The adapter driver then queues these commands to the adapter, providing that the adapter supports command tag queuing. If the adapter does not support command tag queuing, then the adapter driver sends only one command at a time to the adapter and so multiple commands are not queued to the disk.